Eliminate the rev limiter?

[Mike Cash]

If you don't short shift you are missing out on the best power production from this engine. This engine is designed to produce torque, not horsepower. The best wave of torque to surf is well below redline. Wringing it's neck for more power will only disappoint.

Try it - you'll like it.

I normally shift around 3,000. My comment had to do with the designed behavior of the rev limiter, not my shifting habits. The first time I hit the rev limiter was passing on a two-lane road and that sudden power cutoff damned near made me sh!t my pants. Far more alarming than if it had just gone to 6,500 and stayed there refusing to go higher.

 

[Mojave]

Last weekend I did a 1000 mile ride through the windy backroads of central California - Ridgecrest to San Francisco and back. Content was about 300 miles freeway (101, 280), 300 miles nice secondary roads, and 400 miles paved stagecoach roads - the kind too narrow to have a center strip. I had lots of passing opportunities, and the NC basically did fine. Since I was on unfamiliar roads I think my riding was a little more, uhem, conservative. So, I wasn't in my usual slashing mode where I know every curve and every straight.

I think I hit the limiter a couple times, more just forgetting to shift than really trying to accelerate even harder. Once, tooling south on 101 at 80mph, after riding into it from the amazing Carmel Valley Road, I looked down and wondered why the tach was at 5,000rpm - smooth engine!

The only time I contested a car was on the 101. Some dude in the left lane was basically blocking the flow by tracking a car going about 70 in the right lane. I came up on this knot of six or seven cars, easily weaved through till going around (without splitting) the front two. At that the guy sped up to about 90 trying to stay with me. I held at 95 till there was a few hundred feet between us and he broke off to go back to his traffic control weirdness.

A couple on non-rev limiter observations. I averaged 60mpg for the trip. The stock seat was fine for me in 21 hours of riding. Grooved pavement and turbulent air from behind trucks creates moderately unsettled tracking. The front forks really need help for rough pavement.

 

[davidc83]

I normally shift around 3,000. My comment had to do with the designed behavior of the rev limiter, not my shifting habits. The first time I hit the rev limiter was passing on a two-lane road and that sudden power cutoff damned near made me sh!t my pants. Far more alarming than if it had just gone to 6,500 and stayed there refusing to go higher.

I did hit the rev limiter once on my BMW G650GS, I was passing a semi on a 2 lane highway, dropped down to 4th (only has 5 gears), twisted the throttle and actually forgot to upshift to 5th once I was 80+mph-I was concentrating too hard on the semi driver erratic behavior. Bike started bucking like a bronco and I thought 'what is wrong with my bike', I was doing 85 in 4th and upshifted and everything was cool.

 

[bigjeff]

I normally shift around 3,000. My comment had to do with the designed behavior of the rev limiter, not my shifting habits. The first time I hit the rev limiter was passing on a two-lane road and that sudden power cutoff damned near made me sh!t my pants. Far more alarming than if it had just gone to 6,500 and stayed there refusing to go higher.

You have more torque than horsepower up to ~ 5,000 rpm. Revving higher than 5,000 rpm will not make you accelerate any faster than shifting to a higher gear and letting that torque pull harder at the new gear ratio.

The extra horsepower available after 5,000 rpm may pull you up a steep hill faster, but for real world riding, the torque available between 3-5000 rpm is all you need. (see chart)

Ahhhh the qualities of a stroker engine...

 

[Mike Cash]

I wasn't trying to accelerate faster....I was trying to GO faster. Big difference.

 

[L.B.S.]

You have more torque than horsepower up to ~ 5,000 rpm. Revving higher than 5,000 rpm will not make you accelerate any faster than shifting to a higher gear and letting that torque pull harder at the new gear ratio.

The extra horsepower available after 5,000 rpm may pull you up a steep hill faster, but for real world riding, the torque available between 3-5000 rpm is all you need. (see chart)

Ahhhh the qualities of a stroker engine...

I discovered an interesting thing when I was racing, many years ago. I was on my trusty CB450T Hawk (1982 180* parallel twin)

Most of the track, I would get completely blown into the weeds by all the other bikes in the class- Unlimited Twins, (or BOTT as it was called then)


But lo and behold, I found if I stayed in 5th gear for waaay longer than "normal" before shifting into 6th, that I could keep a fair number of bikes from having the speed to overtake me, on surprising sections. (I only found this out by accident, when once I thought I was in 6th already lol)

I had a few guys ask me what it was that I had done to "up my bikes power" so that I could go as fast as them all of a sudden, when before they would easily walk past. Of course I didn't tell them that they were shifting too early into their top gear, while I was simply maintaining my rpm in 5th. (The reason I was already in front of them in the first place, was, it was way easy for me to outbrake and pass them on the inside or around the outside.)

I had a super flashback to the exact shifting too early thing, when I bumped into the rev limit wall on the NC a few times trying to accelerate into traffic from a side street.

I wanted to get from A position to B position, and shifting early seemed to bollox the end result up, not in my favour, right at the threshold of the limiter in first. Going from 1st to 2nd early (or as "apporopriate" for the NCX engine characteristics) it ended up that I was initially slowed more than I wanted by the lag in momentum during the shift, and then accelerating *more* than I wanted to, just when I was expecting to begin the off throttle and braking point. I wanted to hang out in 1st right at the zone where it abruptly shut down. Mucking with sprocket gearing might alter this.


Again, it's not that I want the limit to be higher, just not so abrupt. With nearly all of my other bikes, they either had a super long wind up of rpm so the limiter was seldom hit in the first place, or the gearing was subtly different, so it was a non real world issue. With my CBR125R, it has so little power and such short gearing, that I can't get 1/2 through an intersection without needing to shift into 2nd. I'm constantly at or hitting the rev limiter in all 6 gears, but it's the "soft" style, so it is an easily felt, almost two step process thing, which you can modulate. The NC goes *BANG* dead straight away, and it's an entirely more poor way of doing it, IMHO.

 

[Mojave]

You have more torque than horsepower up to ~ 5,000 rpm. Revving higher than 5,000 rpm will not make you accelerate any faster than shifting to a higher gear and letting that torque pull harder at the new gear ratio.

The units are completely different for the two curves and you can't directly compare torque and horsepower like that. Horsepower is, of course, related to torque - it's torque times RPM times a factor that depends on the units. It's power that is doing the work of overcoming friction and the work of acceleration. Looking at the chart - for best acceleration you need to rev right to the limit and then shift. Just for fun if you multiplied the torque curve by the RPM you will get the horsepower curve but you'd have to scale it to have it near the torque curve. So, normalize the RPM's so that 5000 is 1, 4000 is 0.8, and so on.

 

[670cc]

You have more torque than horsepower up to ~ 5,000 rpm. Revving higher than 5,000 rpm will not make you accelerate any faster than shifting to a higher gear and letting that torque pull harder at the new gear ratio.

The extra horsepower available after 5,000 rpm may pull you up a steep hill faster, but for real world riding, the torque available between 3-5000 rpm is all you need. (see chart)


Ahhhh the qualities of a stroker engine...

View attachment 13212

I think there may be a misunderstanding here about the relationships of torque, RPM, and horsepower. Notice how the torque value for the NC700x is higher than horsepower value when below 5252 RPM? Notice how the lines cross at 5252 rpm? Well every engine in the world is the same way (assuming they actually rev as high as 5252). Check every dyno chart in any motorcycle test and you will see the same thing. The lines always cross at 5252. It's simple math.

We all know that some bikes will pull way harder above 5252 RPM than below it, and some pull harder below than above. The peak power output of some engines happens below 5252 RPM, and on some engines peak power is above 5252. But, the torque value is always higher than the horsepower value below 5252 RPM. That's true of any engine, long stroke or short. Even a classic Ninja 250 that revs to 13,000 RPM, has little bottom end, and needs to be rev'd to get anywhere - same thing.

Torque(lb-ft) x RPM
---------------------- = Horsepower
5252

Looking at the chart, the NC700X horsepower peak is at about 6200 RPM. Based on the gear spacing and the steepness of the HP curve leading up to and falling after the HP peak, you'd probably get maximum acceleration on this bike by shifting right before the rev limit cutout. That way when you row through the gears you're utilizing the most of the hump in the HP curve. Going much beyond the stock rev limit, if it were eliminated, would do no good because the power is falling off at that point. There's probably some statistical way to calculate the mean HP value under the curve in a certain RPM range.

I don't ride that way myself because I'm not in any hurry. I shift at 3000 RPM or below and on partial throttle. I probably never generate more than 15 or 20 horsepower during most of my riding.

Mojave pretty much has it right.

 

[anglachel]

There's probably some statistical way to calculate the mean HP value under the curve in a certain RPM range

not statistics, calculus.

I believe (been a while since I heard all this from the mechanical engineers in college) torque is the first derivative of power, plus a constant (always +C... calc teachers will ding you for leaving that off) that is dependant on the design of the engine. (Not even just the engine but all the hearing comes into play to if you are measuring at the wheel)

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[670cc]

not statistics, calculus.

I believe (been a while since I heard all this from the mechanical engineers in college) torque is the first derivative of power, plus a constant (always +C... calc teachers will ding you for leaving that off) that is dependant on the design of the engine. (Not even just the engine but all the hearing comes into play to if you are measuring at the wheel)

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Got it! Calculus class was too many years back and I never understood it anyway. I guess I would have been safe to just say "mathematical way".

 

[Mojave]

Yes, that's the constant for the units of foot-pounds and horsepower. In my post I should have said normalize 5252 to 1 (divide by 5252), it would have been perfect then. But I didn't because it's easier to visualize the increments of 5000,4000,etc along with the relatively flat torque curve, with then produces that nice linear power "curve". There's some quaint reason for this particular number.

Come on, would somebody please go and get the Wikipedia physics lesson? I'm too tired. And Calculus! 1981-1982, the three classic courses. I can remember what I need for my optical work, 99 percent I'd bet is lost even to my moment of death. Can you imagine, reliving every minute of calculus class just as you are dying? Frankly, if that is what really happens, I hope the replay is perfect because I don't want to go feeling frustrated about not getting some derivation, identity, or algebra trick correct or something.

I don't think the design of the engine is embodied with a constant. It's just force/power/work all the way down. The units or torque/power definitions, need a constant.

 

[showkey]

, you'd probably get maximum acceleration on this bike by shifting right before the rev limit cutout. That way when you row through the gears you're utilizing the most of the hump in the HP curve. .

Think you are right.......and suspect it is not a coincidence the rev limiter is placed right above this max acceleration rpm...

 

[Fuzzy]

You have more torque than horsepower up to ~ 5,000 rpm. Revving higher than 5,000 rpm will not make you accelerate any faster than shifting to a higher gear and letting that torque pull harder at the new gear ratio.

The extra horsepower available after 5,000 rpm may pull you up a steep hill faster, but for real world riding, the torque available between 3-5000 rpm is all you need. (see chart)


Ahhhh the qualities of a stroker engine...

View attachment 13212

This analysis on forums almost always misunderstands RPM. The power from the engine to the wheel is a combination of torque and rpm. If you slow the rpm proportionally more than the torque is reduced you will have less power at rear wheel which is why the hp curve starts to drop, but not until above 6000 rpm. Waiting until 6000 rpms to change gears will give fastest acceleration and is what my DCT does at WOT even in D mode.

 

[Mojave]

Some physics bits from Wikipedia. And pardon me, I'm not a physicist, I'm a geologist that long ago went into an optics career. BTW, there are bunches of hotrod explanations of torque and horsepower out there.

"The constant 5252 is the rounded value of (33,000 ft·lbf/min)/(2π rad/rev)." *5252 is a normalizing factor*
"Watt found by experiment in 1782 that a 'brewery horse' could produce 32,400 foot-pounds per minute." James Watt and Matthew Boulton standardized that figure at 33,000 the next year."

"Torque, moment or moment of force, is the tendency of a force to rotate an object about an axis,[1] fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist to an object."

"Loosely speaking, torque is a measure of the turning force on an object such as a bolt or a flywheel. For example, pushing or pulling the handle of a wrench connected to a nut or bolt produces a torque (turning force) that loosens or tightens the nut or bolt."

Work is what is done when a force acts on something, displacing it, rotating it, or heating it. If you were trying to turn a huge millstone and the force (torque) you exerted was not enough to overcome the static friction and actually get it moving, from the mechanical standpoint you haven't done any work :^) From the thermodynamic standpoint you know you did some work! A horse can exert enough force to get the thing moving, grinding grist, generating heat, etc. And keep it moving, generating about 10 times the work of an average man - horse has about 10 times the "horse" power of a man, and can do ten times the work. Needs ten times the hay too.

A light blub is doing work consuming electrical power and converting it into light and heat.

So power is energy transfer - the amount of Watts (Joules per second) that is available to do work. The more power available, the faster work can is done. A motorcycle only needs work done to do two main things - overcome frictional power losses and accelerate. Friction at low speeds is from things like bearings, internal friction in the tires, internal friction in the engine and drivetrain, and all of this is relatively low compared to the amount of power available. As speed increases aero friction increases exponentially and dominates, becoming the limiting factor. Acceleration is the condition of creating ever increasing speed - it takes some work done per unit of increased speed. The faster the acceleration - the less time per unit of increased speed, the more power required. Integrals galore! Kind of equivalent to acceleration is holding a constant speed on an uphill grade, though the aero friction remains the same. Likewise holding a constant speed on the level against frictional losses, this is a kind of acceleration. Combining actual acceleration and aero friction and grade in one expression gets you into that next math class, differential equations.

 

[670cc]

If one was bored, one could read more about horsepower and the misconceptions about torque in this thread: http://nc700-forum.com/forum/nc750-general-discussion/4802-bhp-post78991.html?highlight=Torque+horsepower#post78991 and others.

We beat this topic up from time to time.

 

[Cagedodger]

I've skipped most of this thread because of the topic (it really does come up seasonally) so I apologize if this has already been brought up or beaten as if it were a dead horse:
The gearing of the transmission plays a very important part in milking the engine output. The gearbox on the NC7 is geared low and it makes the most use out of the low, but extremely fast, revving torquey bastard that we straddle. When I'm really on the throttle, which is rare because I had no intention of buying a race bike, I can feel the acceleration fade when I'm near the red line. Removing the red line wouldn't make the bike faster, just less reliable.
To appreciate the NC7, you have to understand the design concept. It's a gas sipper with upright ergonomics and a very balanced posture. Jack of all, master of none. For such an efficient bike, it is very quick. I get it and I love it.

 

[bigjeff]

rpm builds faster in the max torque range, builds way slower in the max hp range. Bike does not have the lungs or injectors to rev high.

 

[Mojave]

rpm builds faster in the max torque range, builds way slower in the max hp range. Bike does not have the lungs or injectors to rev high.

I don't disagree with the first part of your statement. The second part I know what you mean but it doesn't bear on torque and power.

For some starting speed,acceleration will be greatest when the bike is in the gear that has the engine at the maximum horsepower RPM.

Here is an example of why this is the case. For this example to be clear it's best to eliminate the drastically changing aero drag that comes from going faster. Going up a grade is the same as acceleration - you are fighting the force of gravity that would accelerate you to the tune of 10meter/sec/sec in freefall. So lets say you are going up a grade at 83mph in sixth gear pulling 4500 RPM and you are WFO, maxxed, pinned on the throttle. All of the POWER that is available at that RPM is being used to overcome aero friction and the grade - no RPM building, no "acceleration". By shifting to 5th the engine is now spinning 5600 and is now producing less torque but more power, the bike will accelerate (RPM's are "building"), until the power available at some RPM higher than 5600 is balanced against the higher frictional losses and the acceleration of gravity. This is a phenomena we all have experienced. This is why we downshift when fighting a headwind, go up a grade, or need to overtake a car. We downshift to get more power.

 

[670cc]

You know, frankly, I wish the people that make these dyno charts would stop putting both the torque and the horsepower curves on them. When the chart shows only horsepower on the vertical axis and RPM on the horizontal, it tells you everything you need to know. Then people might stop saying torque does this but horsepower does that. The bottom line is that power does everything.

 

[bigjeff]

Ok. That all makes sense.

Now, on a flat road - (Florida is all flat roads it seems) I can accelerate faster short shifting than if I run to redline in every gear. Why is that?